BARRIER OR BUFFER MATERIALS.

Why use barriers when welding?

There are many reasons for using barriers and many different barriers.

Basically a barrier reduces the capacitance between the die and the bed plate allowing more area to be welded. It also reduces thermal loss.

When welding thin material a thin barrier must be used, otherwise too much voltage will be generated across the barrier, perhaps causing breakdown.

Supported materials can be welded without a barrier if the tooling is accurate and no more than 20cm ²/ KW is welded, also the support backing acts as a depth stop.

By suitable power setting and weld pressure, clear pockets can be welded to a completed binder with cardboard in position.

No barrier is used in this case as the cardboard acts as a barrier.

As the term " barrier " suggests a suitable insulating material placed between the electrodes, provides thermal and electrical insulation between the die and the lower plate, or upper plate if the die is placed on the lower plate.

A barrier material has the following variable properties.

* Thickness

* Dielectric constant

* Loss factor

* Voltage breakdown resistance

* Surface texture

* Operating temperature without degradation

* Hardness and resistance to cold flow

* Non stick properties

-- Most of these variables effect life of the barrier.

The H.F. voltage between the die and the plate is divided between the barrier and plastic sheeting so if voltage breakdown point of either is exceeded, an arc through will occur.

A material like bakelite assists the welding process when tear seal welding by minimising heat loss, and preventing the knife edge shorting to the base at the completion of the weld cycle. If a barrier is too thick too much voltage will be dropped across the barrier, and insufficient voltage will be available for the weld. Also the H.F. field pattern will be much wider, causing stray field heating of plastic at the sides of the weld and possible thinning of this plastic. This stray field will also take more power from the generator.

Sometimes surface-arcing can occur across the surface of a thick barrier ( over .030 " ) without puncturing the barrier. This is because of a corona effect, possibly off a sharp edge.

Dielectric constant on most materials will vary from 1.5 - 5.5 and will affect the ratio of the voltages across plastic and barrier.

Loss factor will cause temperature rise and this factor can be used for hard to weld materials. A type of heat weld takes place. Nylon can be welded easier in this way.

As H.F. welding potential's can approach 50 - 90% of breakdown voltage of the barrier when hot, insulation resistance is most important.

Surface texture can be very important when welding embossed or textured surfaces. If a gloss surface is detrimental to appearance a barrier such as glasscloth, empire cloth or PTFE filled glasscloth, will give an emboss with a cloth texture appearance.

Maximum operating temperature is very important, for instance, ordinary bakelite sheet may last 10 welds, but 1st grade natural bakelite may last 5000 welds or more. Degradation and delamination is visible and can be seen, by change in colour.

High temperature materials include PTFE, glasscloth, silicone rubber and PTFE filled glasscloth ( tygaflor fluroglass ).

The most impressive effect of hardness we have seen was in an automatic machine with a continuous barrier.

The barrier used in this case was clear Mylar and the pressures and frequency used caused cold flow and slight impressions in the barrier on each weld. As these indentations appeared at random under the die, slight air gaps occurred, causing arcing.

This machine operated on 80 MHZ and the problem was solved by using PTFE filled glasscloth.

WELDING WITHOUT BARRIERS.

Bar welding, as in tarpaulins, pool liners ect., is often carried out with no barrier.

Advantages in this case are :-

* Stronger weld.

* Faster welding.

* Much faster more positive arc suppressor operation.

* Automatic weld termination.

* Less problems with sticking when electrodes separate.

Arc suppressors of the resistance change type cannot operate without a conductive path through the barrier. Without a barrier, arcs are generally stopped with no damage at all.

The arc suppressor needs a control for reducing sensitivity, as a bar say 800mm long and 10mm wide would cause a hot plastic resistance of less than 250 K ohm.

This drop in resistance is used to switch off the H.F. Generator sometimes at a certain point in each cycle.

A stronger weld can be achieved, because less stray field heating takes place at the edge of the electrode, so there is less weakening caused by tensions and accidental pulling at the weld edges while they are still in plastic , or molten state.

Faster welding is achieved, because stored heat at weld interfaces is allowed to escape faster, back through the cold metal electrodes and because sheeting adjacent to the weld area is cold, heat is conducted quickly away.

It must be stressed that when a barrier is used the barrier is itself heated by H.F. energy, the plastic received heat from underneath. This means that greater areas can be heated when a barrier is used.

A desirable and most useful barrier material is elephantite + Mylar .012 + .003.

This material combines the loss factor of elephantite or prespahn with the smoothness and heat properties of Mylar.

Without the elephantite the mylar exhibits a tendency to "cold flow" and thus have less dimensional stability.

Barrier materials commonly used with H.F. welders are : -

* Elephantite + Mylar .012 + .003. These figures refer to the thickness of the laminates.

* Mylar.

* 1st grade natural bakelite .

* PTFE ( teflon ) impregnated fibreglass.

* Capton. A thin high temperature material used on autos as a slip barrier.

* Empire cloth ( now superseded and little used .)